Transient voltage suppressor (TVS) with improved clamping voltage

ABSTRACT

This invention discloses an electronic device formed as an integrated circuit (IC) wherein the electronic device further includes a transient voltage suppressing (TVS) circuit for suppressing a transient voltage. The transient voltage suppressing (TVS) circuit includes a Zener diode connected between a ground terminal and a node for triggering a snapback circuit. In one embodiment, this node may be a Vcc terminal. The TVS device further includes a snapback circuit connected in parallel to the Zener diode for conducting a transient voltage current with a snapback current-voltage (I-V) characteristic upon turning on of the snapback circuit And, the TVS device further includes a snapback suppressing circuit connected in series with the snapback circuit for conducting a current with an I-V characteristic complementary to the snapback-IV characteristic for clamping a snapback voltage.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part Application of a patentapplication Ser. No. 12/454,333 filed May 15, 2009 now U.S. Pat. No7,933,102, by a common inventor of this application, which is aDivisional of patent application Ser. No. 11/444,555, filed May 31,2006, by a common inventor of this application, now U.S. Pat. No.7,538,997. The disclosures made in the patent application Ser. No.12/454,333 are hereby incorporated by reference in this patentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a circuit configuration and method ofmanufacture of a transient voltage suppressor (TVS). More particularly,this invention relates to an improved circuit configuration and methodof manufacturing a transient voltage suppressor (TVS) with improvedclamping voltages.

2. Description of the Relevant Art

The transient voltage suppressors (TVS) are commonly applied forprotecting integrated circuits from damages due to the inadvertentoccurrence of an over voltage imposed onto the integrated circuit. Anintegrated circuit is designed to operate over a normal range ofvoltages. However, in situations such as electrostatic discharge (ESD),electrical fast transients and lightning, an unexpected and anuncontrollable high voltage may accidentally strike onto the circuit.The TVS devices are required to serve the protection functions tocircumvent the damages that are likely to occur to the integratedcircuits when such over voltage conditions occur. As increasing numberof devices are implemented with the integrated circuits that arevulnerable to over voltage damages, demands for TVS protection are alsoincreased. Exemplary applications of TVS can be found in the USB powerand data line protection, digital video interface, high speed Ethernet,notebook computers, monitors and flat panel displays.

FIG. 1A-1 shows a typical commercially available two-channel TVS array10. There are a pair of steering diode, i.e., diodes 15-H and 15-L, and20-H and 20-L respectively for each of the two input/output (I/Os)terminals I/O-1 and I/O-2. Furthermore, there is a Zener diode, i.e.,main diode 30, with a larger size to function as an avalanche diode fromthe high voltage terminal, i.e., terminal Vcc, to the ground voltageterminal, i.e., terminal Gnd. At a time when a positive voltage strikeson one of the I/O pads, the high side diodes 15-H and 20-H provide aforward bias and are clamped by the large Vcc-Gnd diodes, e.g., theZener diode 30. The steering diodes 15-H and 15-L and 20-H and 20-L aredesigned with a small size to reduce the I/O capacitance and therebyreducing the insertion loss in high-speed lines such as fast Ethernetapplications. FIG. 1A-2 shows the reverse current IR versus reverseblocking voltage V_(R) characteristics between the Vcc and the groundvoltage of the TVS array 10 shown in FIG. 1A-1. The reverse current IRas that shown in the diagram of FIG. 1A-2 represents a reverse currentconducted through the Zener diode, i.e., between Vcc and GND. Here it isassumed that the reverse breakdown voltage (BV) of each steering diodeis higher than the reverse BV of the Zener diode. But note that at highcurrents when the Vcc to Gnd pad voltage is equal or higher than thesummation of the reverse BV of the steering diodes then the currentwould also flow through all the two series steering diode paths. Sincethe Zener diode has higher resistance per unit area compared withbipolar junction transistor (BJT) or semiconductor controlled rectifier(SCR) and BJT this is actually a disadvantage at higher currents becausethe steering diodes also have to be rugged in reverse conduction. In thecase of the SCR+BJT the Zener clamp voltage is lower at higher currentsand hence the steering diodes paths will not conduct. The breakdownvoltage of the Vcc-Gnd diode 30 and the steering diodes 15 and 20 shouldbe greater than the operating voltage (VRWM) so that these diodes onlyturn-on during the voltage transients. The problem with the Vcc-Gndclamp diodes is that typically these diodes are very resistive inreverse blocking mode and require large area to reduce resistance. Asshown in FIG. 1A-2, the high resistance leads to the increase of BV athigh current. This is not desirable as high BV not only causes the breakdown of steering diodes as described above but also causes damage to thecircuit the TVS device intends to protect. The requirement to have largediode size thus limits further miniaturization of a device when such TVScircuit is implemented.

One common method used in the integrated circuits to circumvent thisdrawback is to use a Zener triggered NPN as the clamp device as thatshown in FIG. 1B-1. The TVS circuit 50 shown in FIG. 1B-1 comprises aNPN bipolar transistor 55 connected in parallel to a Zener diode 60 tofunction as a Zener triggered NPN bipolar TVS device. FIG. 1B-2 shows acurrent-voltage (IV) diagram for the Zener triggered NPN diode device.FIG. 1B-2 illustrates that when the collector voltage of the NPN 55reaches the breakdown voltage BV of the Zener diode 60, the NPN bipolarturns-on and snaps back to a lower voltage called the BVceo or holdingvoltage of the NPN bipolar transistor 55 where BVceo stands forcollector to emitter breakdown voltage with base left open. However, ina device that implements a TVS circuit, the snap-back phenomenon is notdesirable. The snap-back creates a sudden drop of the reverse voltagethat often causes the circuit oscillations due to negative resistance,or can even drop into the operating voltage of the device the TVScircuit is supposed to protect.

In order to resolve the snap-back difficulties, a patent applicationSer. No. 12/454,333, of which this application is aContinuation-in-Part, was previously submitted on May 15, 2009 by acommon inventor of this application. The disclosures made in thatApplication are hereby incorporated by reference in this patentapplication. The TVS circuits as described in the patent applicationSer. No. 12/454,333 are implemented with a semiconductor controlledrectified (SCR) circuit to reduce the voltage-drop in a reverse-blockingvoltage snap-back when a reverse current transmitted over a Zener diodetriggers and turns on an NPN bipolar transistor. The TVS circuitsdisclosed in the patent application Ser. No. 12/454,333 resolve thedifficulties caused by increasing break down voltage due to TVS deviceresistance and drastic voltage drop due to a snapback that commonlyoccurs in the conventional TVS circuit. Another aspect of applicationSer. No. 12/454,333 was to provide an improved device design andmanufacturing method to provide an improved TVS circuit. Furthermore,the TVS invention in the patent application Ser. No. 12/454,333disclosed a new TVS for integration into mainstream complementary metaloxide semiconductor (CMOS) or Bipolar-CMOS (Bi-CMOS) technology allowinga configuration of single chip TVS protected ICs thus reduced the costsfor providing the protected ICs on the electronic devices.

FIG. 1C illustrates the improved snapback profile of the SCR TVS circuitcompared to the prior art. However, the reduced snapback shown in patentapplication Ser. No. 12/454,333 is achieved through elaborate layoutconfigurations and difficult optimizations which can be difficult todesign and implement, therefore there is still a need to further improveand simplify the clamping voltage of the TVS device.

Therefore, a need still exists in the fields of circuit design anddevice manufactures for providing a new and improved circuitconfiguration and manufacturing method to resolve the above-discusseddifficulties. Specifically, a need still exists to provide new andimproved TVS circuits that occupy small areas, eliminate or reduce thesnapback voltage variations and furthermore that the TVS circuit canfurther improve the clamp voltage.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide animproved TVS circuit to have an improved clamping voltage to furtherimprove TVS protection such that the above discussed limitations anddifficulties can be resolved.

Another aspect of this invention is to provide TVS protection circuit bycombining two components that include a forward biased diode and asemiconductor controlled rectifier (SCR) to reduce the snap back andalso to improve the voltage clamping performance.

Another aspect of this invention is to provide TVS protection circuit byconnecting a snapback suppressing diode with a snapback circuit inseries such that the snapback suppressing diode provide acurrent-voltage (I-V) characteristic complementary to an I-Vcharacteristic of the snapback circuit such as an SCR circuit such thatthe snapback is substantially reduced.

Another aspect of this invention is to provide TVS protection circuitwith a high side steering diode functioning as a snapback suppressingdiode to operate together with an SCR circuit functioning as a snapbackcircuit such that the snapback suppressing diode complements the I-Voperational characteristics during a transient snapback event that thesnapback voltage is substantially reduced and a clamping voltageaccording to the I-V characteristic.

Briefly in a preferred embodiment this invention discloses a transientvoltage suppressing (TVS) circuit for suppressing a transient voltage.The transient voltage suppressing (TVS) circuit includes a Zener diodeconnected between a ground terminal and a node for triggering a snapbackcircuit. In one embodiment, this node may be a Vcc terminal. The TVSdevice further includes a snapback circuit connected in parallel to theZener diode for conducting a transient voltage current with a snapbackcurrent-voltage (I-V) characteristic upon turning on of the snapbackcircuit. And, the TVS device further includes a snapback suppressingcircuit connected in series with the snapback circuit for conducting acurrent with an I-V characteristic complementary to the snapback-IVcharacteristic for clamping a snapback voltage. In an exemplaryembodiment, the snapback circuit further comprising a semiconductorcontrolled rectifier (SCR). In another exemplary embodiment, thesnapback suppressing circuit further comprising a snapback suppressingdiode connected in series with the snapback circuit In another exemplaryembodiment, the snapback suppressing diode is forward conducting whenconducting a transient voltage current. In another exemplary embodiment,the snapback circuit further comprising a NPN bipolar transistorconnected with a PNP bipolar transistor thus forming a PNPNsemiconductor-controlled rectifier (SCR) structure. In another exemplaryembodiment, the snapback suppressing circuit further comprising asnapback suppressing diode connected in series between a Vcc terminaland the node. In another exemplary embodiment, the TVS device furtherincludes a triggering diode connected to the snapback circuit; and thesnapback circuit further comprising a NPN bipolar transistor connectedwith a PNP bipolar transistor thus forming a PNPNsemiconductor-controlled rectifier (SCR); wherein the triggering diodehaving a breakdown voltage BV for triggering the SCR and the BV of thetriggering diode is less than or equal to a BVceo of the NPN bipolartransistor where the BVceo stands for a collector to emitter breakdownvoltage with the base left open represented by an equation of BV(Trigger Diode)≦BVceo. In another exemplary embodiment, the TVS devicefurther includes an input/output (I/O) pad connected to the node througha high side steering diode and the I/O pad connected to the groundterminal through a low side diode. In another exemplary embodiment, theTVS device further includes an input/output (I/O) pad connected to thenode through a high side steering diode and the I/O pad connected to theground terminal through a low side diode wherein the high side diodefurther functioning as a snapback suppressing diode for the snapbackcircuit for triggering the snapback circuit. In another exemplaryembodiment, the snapback circuit further comprising a NPN bipolartransistor connected with a PNP bipolar transistor thus forming a PNPNsemiconductor-controlled rectifier (SCR) wherein the triggering diodehaving a breakdown voltage BV for triggering the SCR and the BV of thetriggering diode is less than or equal to a BVceo of the NPN bipolartransistor where the BVceo stands for a collector to emitter breakdownvoltage with the base left open represented by an equation of BV(Trigger Diode)≦BVceo. In another exemplary embodiment, the TVS devicefurther includes a first input/output (I/O) pad and a second I/O padconnected to the Vcc terminal through a first and a second high sidesteering diodes respectively and the first and second I/O pads connectedto the ground terminal through a first and a second low side steeringdiodes respectively wherein the first and second high side diode furtherfunctioning as snapback suppressing diode for the snapback suppressingcircuit. In another exemplary embodiment, the snapback circuit furthercomprising a NPN bipolar transistor connected with a PNP bipolartransistor thus forming a PNPN semiconductor-controlled rectifier (SCR)wherein the triggering diode having a breakdown voltage BV fortriggering the SCR and the BV of the triggering diode is less than orequal to a BVceo of the NPN bipolar transistor where the BVceo standsfor a collector to emitter breakdown voltage with the base left openrepresented by an equation of BV (Trigger Diode)≦BVceo. In anotherexemplary embodiment, the snapback suppressing circuit furthercomprising a plurality of diodes connected in series and the pluralityof diodes connected in series between the Vcc terminal and the snapbackcircuit.

This invention further discloses a method for reducing the snapback of acircuit by placing a snapback suppressing diode in series with thecircuit. The I-V characteristics of the snapback suppressing diode canbe combined with the I-V characteristics of the circuit to reduce thesnapback in the circuit. The snapback suppressing diode may be forwardconducting. The resistance of the diode can be adjusted to change itsI-V characteristics. Also, multiple snapback suppressing diodes may beplaced in series.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1 is a circuit diagram for showing a conventional TVS device andFIG. 1A-2 is an I-V diagram, i.e., a current versus voltage diagram, forillustrating the reverse characteristics of the TVS device.

FIG. 1B-1 is a circuit diagram for showing another conventional TVSdevice and FIG. 1B-2 is an I-V diagram for illustrating the reversecharacteristics of the TVS device with the voltage presents a suddensnap-back voltage drop at the time when a current conduction over theNPN bipolar transistor is triggered.

FIG. 1C is an I-V diagram for illustrating an improved snap-backreduction TVS circuit as that disclosed in patent application Ser. No.12/454,333 filed by an inventor of the present invention.

FIG. 2A is a circuit diagram for showing a TVS circuit of this inventionand FIG. 2B is an I-V diagram for illustrating the reversecharacteristics of the TVS device with significantly reduced snap backvoltage drops and further improved voltage clamping.

FIGS. 3A to 3C are side cross sectional views of circuit components ofthe TVS device of FIG. 2A supported on a semiconductor substrate for asnapback suppressing diode, low side diode, and a high side dioderespectively.

FIG. 4 is a circuit diagram for showing a TVS circuit implemented with ahigh diode functioning as a snap back suppressing diode without externalconnection to a Vcc terminal.

FIG. 5 is a circuit diagram of another TVS device implemented with asnapback circuit combined with a snapback suppressing circuit such as adiode to achieve reduced snapback and improved clamp voltage.

DETAILED DESCRIPTION OF THE METHOD

FIGS. 2A and 2B depict respectively a circuit diagram and an I-Vdiagram, i.e., a current versus voltage diagram, of a TVS circuit 100 ofthis invention. The TVS circuit 100 is installed between a groundvoltage terminal (Gnd) 105 and a Vcc voltage terminal 110 to function asa Vcc-Gnd clamp circuit. The TVS circuit 100 includes a pair of steeringdiodes, i.e., diodes 115-H and 115-L, and 120-H and 120-L respectivelyfor each of the two input/output (I/Os) terminals 125-1 and 125-2.Furthermore, there is a Zener diode, i.e., diode 130, with a larger sizeto function as a trigger diode from the high voltage terminal, 110 i.e.,terminal Vcc, to the ground voltage terminal, i.e., terminal Gnd. TheZener diode 130 is connected in series with a resistor 135 and inparallel to a PNP bipolar transistor 142 and NPN bipolar transistor 140.The PNP bipolar transistor 142 is configured with a NPN bipolartransistor 140 to form a PNPN silicon-controlled rectifier (SCR)structure 150 with high holding current and voltage. The TVS circuit 100further includes a snapback suppressing diode 155 connected in seriesthrough a node 118 for interconnecting between the Vcc terminal 110 andthe SCR 150. The node 118 also functions as a common node connecting tothe high side steering diodes 115-H and 120-H. The breakdown voltage,i.e., BV, of the triggering diode 130 is less than or equal to the BVceoof the NPN bipolar transistor 140 or PNP bipolar transistor 142 whereBVceo stands for collector to emitter breakdown voltage with the baseleft open.BV(Trigger Diode)≦BVceo

FIG. 2B is a current versus voltage diagram for the operationalcharacteristics of the TVS according to FIG. 2A. As a transient voltagehigher than a normal operating voltage is applied to the TVS circuit, areverse current is triggered to pass through the Zener diode 130 becausethe breakdown voltage BV of the trigger diode 130 is adjusted to avoltage that is less than the BVceo. As the voltage increases, thedevice migrates into BJT mode where the NPN 140 conducts. When thevoltage increases further the SCR 150 is activated and begins to conductcurrent. The turning on of the SCR causes a drop of the reverse blockingvoltage V_(R) while the SCR combined with the diode current component,i.e., the SCR-Diode component, negates some of the snapback to keep thevoltage substantially at or above the operating voltage thus providing asignificantly improved voltage clamping performance.

Specifically, FIG. 2B shows the I-V diagrams, i.e., curve 160 for thesnapback suppressing diode 155, wherein the snapback suppressing diodeis configured for conducting the transient voltage current in forwardconduction mode. It also shows the curve 165 of a typical TVS devicesimilar to TVS device 100, but lacking the snapback suppressing diode155. The curve 165 exhibits a large snapback, and can even enter theoperating voltage of the protected device, which is highly undesirable.If the snapback voltage is lower than the operating voltage, it canundesirably short the supply voltage Vcc during normal operatingvoltages. The circuit schematic represented by curve 165 may look likeone shown in U.S. application Ser. No. 12/454,333, but itsimplementation does not have elaborate layouts schemes to suppress thesnapback, and thus exhibits a large snapback. The curve 170 shows theI-V behavior of the TVS device 100 (including snapback suppressing diode155). A portion 170-1 of the curve 170 shows the I-V characteristics ofthe BJT 140 before the SCR 150 is turned on and the curve 170-2 showswhen the SCR 150 is turned on. Instead of using complex layout schemesto suppress the snapback, the TVS device 100 employs the snapbacksuppressing diode 155 to alleviate the snapback. In addition tosuppressing snapback in the TVS device 100, the snapback suppressingdiode 155 is also able to improve the clamping characteristics of thecurve 170, which displays a more vertical profile upon clamping than theold curve 165. In an alternative embodiment, the snapback suppressingdiode 155 may be configured to be in reverse blocking mode whenconducting a transient voltage current. The purpose of the snapbacksuppressing diode 155 is to shape the I-V characteristics of the TVSdevice 100 to suppress snapback, and improve clamping.

By adjusting the BV of trigger diode 130 less than the BVceo and greaterthan the operating voltage, the voltage is snapped back at when the SCRis turned on while the SCR-Diode voltage is clamped at substantially aconstant voltage above the operating voltage. The BV of the triggerdiode 130 may be easily adjusted to the desired voltage for triggeringthe 170-1 portion of the curve 170, even with the addition of thesnapback suppressing diode 155. The clamping voltage performance istherefore significantly improved.

The sequence of operation mode provides the benefit of fast response asthe NPN transistor turns on fast Furthermore, by turning on the SCR 150at a SCR trigger voltage to protect the NPN bipolar transistor 140, theincrease of BV at high current is minimized as the SCR action incurs theleast resistance. This resolves the difficulties of high BV at highcurrent that not only causes the break down of steering diodes but alsocause damage to the circuit the TVS device intends to protect.

The detail operation of the TVS system can be further understood fromthe following descriptions. Typically the TVS is biased in a system withthe high voltage terminal Vcc and the ground voltage Gnd connected tothe system which needs protection. There are also applications where theVcc is left floating for specific applications. Then a +Ve or −Ve zap isapplied to the I/O terminals with respect to Gnd. When a +Ve zap isapplied on I/O the upper diodes are forward biased and when the voltagereaches the trigger diode BV then current flows through the resistor 135in series with the trigger diode 130. When the drop in the resistor 135reaches a voltage of about 0.6V then the base-emitter junction of theNPN transistor 140 is forward biased and the NPN transistor 140turns-on. Now the collector current of the NPN transistor flows throughthe resistor connected between the emitter and base of the PNPtransistor 142. When the potential drop in this resistor 145 reaches avoltage of 0.6V then the emitter of the PNP transistor 142 begins toconduct and the SCR action is initiated. So now the current flows fromthe anode on the PNP 142 transistor, i.e., the emitter of PNP, to thecathode of the NPN transistor 140, i.e., the emitter of the NPN. On thenegative zap the bottom diode turns-on in forward conduction between I/Opad and Gnd and ESD current flows only in this diode path. There is alsoa condition when a voltage zap is applied to the Vcc at a voltage of +Vewith respect to Gnd. Under this zap condition the current flows throughthe Vcc-Gnd path, i.e., there is no current conducted in the steeringdiodes, since the trigger diode breaks down and initiates the SCR asdescribed above. The SCR itself may suppress the snapback slightlycompared to a conventional device, but the snapback suppressing diode155 will shape the I-V characteristics of the device to further suppresssnapback and to improve the clamping I-V characteristics.

FIGS. 3A to 3C are cross sectional views showing the TVS array thatincludes the trigger diode 130 integrated with NPN bipolar transistor140 and the PNP bipolar transistor 142 forming SCR 150, and two pairs ofsteering diodes 115-L, 115-H, and 120-L, 120-H (only one pair isillustrated). The TVS device further includes a snapback suppressingdiode 155 connected in series between the Vcc terminal 110 and the SCR150. The new TVS array 100 as shown in FIGS. 3A to 3C is manufacturedwith mainstream CMOS technology. FIG. 3A shows a TVS array supported ona P substrate 200. A P type region 210 is placed next to an N+ region215 forming a Zener diode 130 with cathode connecting to a metal node118. A P+ region 220 also connects to the metal node 118 and the. P+region 220 is disposed next to an N well region 230 above P substrate200 forms PNP transistor 142, with P substrate 200 connecting to Gnd pad105 through P well 240 and P region 242. The lateral path in P substrate200 from N well 230 to P well 240 provides the resistance for resistor135. The path from N region 235 to N well 230 provides resistance 145.The N-well 230 disposed above the P-substrate 200 in turn electricallycontacting an N-region 245 thus constituting the NPN transistor 140. TheP-type region 210 formed next to the N+ region 215 within P well 240 isto tailor the trigger breakdown voltage BV of the trigger diode 130,i.e., the diode formed between the P region 210 and the N+ region 215,to be less than or equal to the BVceo of the NPN transistor 140. Theother way of tailoring the BV and BVceo is to increase the gradient ofthe N doping of N+ region 215 so that the collector to emitter breakdownvoltage with the base left open (BVceo) is tailored to the desiredvalue. A combination of the two could also be used to get desiredtrigger BV and BVceo. A P+ region 260 is formed near the top surface ofthe N-well 255 at a lateral distance away from the N-well 230. The P+region is connected to a Vcc metal 110 and forming a diode 155 at thejunction of the P+ region 260 interfacing with the N-well 255. The diode155 is connected in series to the Vcc metal 110 and the metal node 118through a N+ region 265 formed on a top portion of the N-well 255. Thediode 155 thus serving the function of snapback suppressing diodeconnected in series between the Vcc terminal 110 and the SCR 150 throughthe common node 118.

FIG. 3B shows the low side steering diode comprises a P+ region 280 andN+ region 285 encompassed in a P-well 290. The low side steering diodecan represent diode 115-L or 120-L. The low side diode's anode isconnected to GND 105 and its cathode is connected to I/O terminal. GND105 of FIG. 3B may be connected to GND 105 of FIG. 3A in the thirddimension. FIG. 3C shows the high side steering diode comprises a P+region 280′ and N+ region 285′ encompassed in a N-well 290′. The highside diode can represent diode 115-H or 120-H, and its anode isconnected to the I/O terminal and its cathode connected to metal node118. The metal node 118 of FIG. 3C may be connected to metal node 118 ofFIG. 3A and the I/O terminal of FIG. 3C may connected to the I/Oterminal of FIG. 3B in the third dimension. For lowering the capacitanceof these diodes and also increasing their BV, a lower doped N− region isadded where N+ region is implanted so the process provides an N+/N−/PWdiode instead of N+/PW diode. Similar for the high side diode, a P−implantation is added where P+ is so as to give a P+/P−/N-well diode.

FIG. 4 is another circuit diagram to show an alternate embodiment of thepresent invention with improved clamp capabilities. The TVS system 100′shown in FIG. 4 has similar circuit configuration as that shown in FIG.2A except that the node 118 is configured as a floating node. That is,the node 118 can be a Vcc pad, but the Vcc is not connected externally.The TVS is not connected to a Vcc through a snapback suppressing diodeas that shown in FIG. 2A and the Vcc pad of this device is not connectedexternally. Therefore, the external current is transmitted into the TVSdevice through the I/O pad 125-1 (or 125-2). The high side steeringdiode 115-H (or 120-H) is functioning as a snapback suppressing diode.The configuration of the TVS device can again be simplified withoutrequiring a specialized layout configuration to suppress the snapback.Instead, the high side diode 115-H is used to adjust the snapbackcharacteristics of the SCR 150.

FIG. 5 is another diagram to illustrate the essence of this invention.Specifically, a TVS device 100″ is connected between a Gnd terminal 105and a Vcc terminal 110. The TVS device 100″ includes a Zener diode 130′for conducting a transient current therethrough. The Zener diode 130′ isconnected in parallel to a snapback circuit 150′, such as a SCR 150shown in FIG. 2A or the NPN bipolar transistor of FIG. 1B-1, forconducting a reverse current with an I-V characteristic to turn on thesnapback circuit 150′ similar to that shown in FIG. 2B. The TVS device100″ further includes a snapback suppressing diode 155′ connected inseries between the Vcc terminal 110 and the snapback circuit 150′ toprovide an I-V characteristic that is complementary to the snapbackcharacteristic of the snapback circuit such that a voltage suppressingperformance during a transient voltage event is significantly improvedby combining the I-V characteristic of the snapback circuit 150′ withthe I-V characteristic of the snapback suppressing diode 155′.

According to FIGS. 3A to 3C, this invention discloses an electronicdevice with the snapback suppressing diode and the SCR TVS device (whichmay include a triggering diode) are integrated with the electronicdevice on a single chip. In a preferred embodiment, the snapbacksuppressing diode and the SCR TVS are manufactured with a standard CMOStechnology and integrated with the electronic device on a single chip.In another embodiment, the snapback suppressing diode and the SCR TVSare manufactured with a standard Bi-CMOS technology and integrated withthe electronic device on a single chip. In another embodiment, the TVSfurther includes a second SCR anode connected in parallel to a first SCRanode of the SCR with the first SCR anode triggering a current throughthe second SCR for further limiting an increase of a clamping voltage.In another embodiment, the TVS further includes high side and low sidediodes. In another embodiment, the snapback suppressing diode, thetriggering and high side and low side diodes, and the SCR are formed ina semiconductor substrate by implanting and configuring dopant regionsof a first and a second conductivity types in a N-well and a P-wellwhereby the TVS can be formed in parallel as part of the manufacturingprocesses of a CMOS or Bi-CMOS electronic device.

With the above circuit diagrams and the device cross sections, theinvention shows the TVS operations and array integration of the improvedTVS devices. These TVS devices provide improved clamp protections thatoccupy smaller area and perform good clamping function because the SCRsare able to carry high currents with little voltage drop beyond triggerdiode breakdown. To one skilled in the art, it should be clear that thisinvention provides a simple and useful method of adding a forwardconducting diode to suppress the I-V snapback of a device. The slope ofthe I-V characteristics of the snapback suppressing diode can be easilyadjusted by changing the resistance of the diode. Multiple diodes can beplaced in series to further modify the I-V characteristics and to reducethe capacitance of the snapback suppressing diodes.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

1. A transient voltage suppressing (TVS) circuit comprising: a snapbackcircuit connected between a ground terminal and a node for conducting atransient voltage current with a snapback current-voltage (I-V)characteristic upon turning on of said snapback circuit; and a snapbacksuppressing circuit connected in series with said snapback circuit forconducting a current with an I-V characteristic complementary to thesnapback-I-V characteristic for clamping a snapback voltage; whereinsaid snapback suppressing circuit further comprising a snapbacksuppressing diode.
 2. The transient voltage suppressing (TVS) circuit ofclaim 1 wherein: said snapback suppressing diode is configured forforward conducting a transient voltage current.
 3. The transient voltagesuppressing (TVS) circuit of claim 1 wherein: said snapback circuitfurther comprising a semiconductor controlled rectifier (SCR).
 4. Thetransient voltage suppressing (TVS) circuit of claim 3 furthercomprising: a Zener diode connected in parallel to said SCR to functionas a triggering diode for said SCR.
 5. The transient voltage suppressing(TVS) circuit of claim 1 wherein: said snapback circuit furthercomprising a NPN bipolar transistor connected with a PNP bipolartransistor thus forming a PNPN semiconductor-controlled rectifier (SCR)structure.
 6. The transient voltage suppressing (TVS) circuit of claim 1further comprising: a Vcc terminal wherein said snapback suppressingdiode is connected in series between said Vcc terminal and said node. 7.The transient voltage suppressing (TVS) circuit of claim 1 furthercomprising: a triggering diode connected to said snapback circuit; andsaid snapback circuit further comprising a NPN bipolar transistorconnected with a PNP bipolar transistor thus forming a PNPNsemiconductor-controlled rectifier (SCR); wherein said triggering diodehaving a breakdown voltage BV for triggering said SCR and the 13V of thetriggering diode is less than or equal to a BVceo of the NPN bipolartransistor where the BVceo stands for a collector, to emitter breakdownvoltage with the base left open represented by an equation of BV(Trigger Diode)≦BVceo.
 8. The transient voltage suppressing (TVS)circuit of claim 1 further comprising: an input/output (I/O) padconnected to said node through a high side steering diode and said I/Opad connected to said ground terminal through a low side steering diode.9. The transient voltage suppressing (TVS) circuit of claim 8 wherein:said high side steering diode further functions as said snapbacksuppressing diode.
 10. The transient voltage suppressing (TVS) circuitof claim 8 further comprising: a triggering diode connected in parallelto said snapback circuit; wherein said snapback circuit furthercomprising a NPN bipolar transistor connected with a PNP bipolartransistor thus forming a PNPN semiconductor-controlled rectifier (SCR)wherein said triggering diode having a breakdown voltage BV fortriggering said SCR and the BV of the triggering diode is less than orequal to a BVceo of the NPN bipolar, transistor where the BVceo standsfor a collector to emitter breakdown voltage with the base left openrepresented by an equation of BV (Trigger Diode)≦BVceo.
 11. Thetransient voltage suppressing (TVS) circuit of claim 1 wherein: saidsnapback suppressing circuit further comprising a plurality of snapbacksuppressing diodes connected in series.
 12. The transient voltagesuppressing (TVS) circuit of claim 10 further comprising: a Vcc terminalwherein said snapback suppressing diode is connected in series betweensaid Vcc terminal and said node and wherein said snapback suppressingdiode is configured for forward conducting a transient voltage current.13. A method of suppressing the snapback of a circuit comprising:placing a snapback suppressing diode in series with a circuit havingsnapback characteristics to suppress the snapback.
 14. The method ofclaim 13 wherein: implementing said snapback suppressing diode to adjustthe current-voltage (I-V) characteristics of said circuit to suppressthe snapback.
 15. The method of claim 13 further comprising: modifyingthe current-voltage (I-V) characteristics of said snapback suppressingdiode by changing the forward resistance of the diode.
 16. The method ofclaim 13 further comprising: connecting multiple snapback suppressingdiodes in series with said circuit having said snapback characteristics.17. The method of claim 13 wherein: configuring said circuit having saidsnapback characteristics comprises configuring a transient voltagesuppressor (TVS) circuit having said snapback characteristics.
 18. Themethod of claim 17 wherein: configuring said circuit having snapbackcharacteristics by implementing a semiconductor controlled rectifier(SCR) and a triggering diode.
 19. The method of claim 17 wherein: saidstep of placing a snapback suppressing diode also improves a currentvoltage (I-V) clamping characteristic of said TVS.
 20. The method ofclaim 17 wherein: said step of placing a snapback suppressing diodefurther comprises configuring said snapback suppressing diode forforward conducting a. transient voltage current.